The actual answer to that is quite fucking complicated actually!
Short answer: Upper-case letters are called majuscules and lower-case letters are called minuscules, but the real answer is that they started out as different scripts entirely, so I'm not sure they were historically called much of anything.
Capitals are (presumably, didn't double-check) named after the Capitalis script; the stuff the ancient Romans hammered into stones. There's also Uncials, which are also an all-majuscule script, as well as some less ubiquitous scripts like the Insular Majuscule. Point is, all of them only had upper-case letters.
These are the oldest non-cursive scripts. Later on, they were supplemented by the invention of space-saving all-minuscule scripts; insular minuscule, carolingian minuscule, the potentially hundreds of other small, regional, pre-caroline scripts.
Those only had lower-case letters.
So, capitalization? Really started by people mixing scripts (and usually ink colors) to visually distinguish parts of a text. Imagine someone put most of a text in Arial and the first letter of each paragraph in.... hm, Papyrus, maybe, and you get the idea. You usually found them at the beginning of verses and chapters and all that stuff. Now, anything after Charlemagne kicked the bucket isn't really my stomping ground, but I'm not actually sure the idea of modern-style capital letters that weren't fancy and red (in which case they're named after their function) actually predates the printing press.
